In a laser of the type employing a flowing gas as the lasing medium, the gas is passed through a discharge cavity where an electric field causes an electric discharge in the gas that produces an emission of light. The lasing of the gas causes the gas to become very hot and the gas flowing out of the discharge cavity, therefore, is at a much higher temperature than the gas entering the discharge cavity. In a closed system where the gas is recirculated, the hot gas from the discharge cavity flows to a heat exchanger where the gas is cooled to restore the population inversion of the gas molecules to an appropriate level that again permits the stimulation of lasing.
In gas lasers of the kind employing CO.sub.2 mixed with N.sub.2 and H.sub.2 as the gas lasing medium, for example, the appropriate length for a straight stable resonant cavity is about 10 meters. Because that length, in many instances, is inconvenient or exceeds available space, the length of the laser has been appreciably reduced by optically folding the resonant cavity. In a folded optical resonator, the length of the cavity is reduced by reflecting the beam between mirrors situated at the ends of the folded cavity. For a folded optical resonator to be effective, precise spacing of the mirrors and their alignment must be accurately maintained.
Because of the heating and cooling of the gas that occurs in the operation of a flowing gas laser, the thermal expansion and contraction of the laser structure makes it difficult to find a stable support for the optical elements of the laser's resonant cavity. Consequently, in flowing gas lasers of moderate or high power, it has been customary to support the optical elements of the resonant cavity on a separate optical bench within or extending through the laser housing.
A flowing gas laser having a separate bench for the support of the optical elements of the laser's resonant cavity is described in U.S. Pat. No. 3,808,553. In that patented arrangement, the bench is constituted by spacer rods made of a material such as Invar steel that has relatively low thermal expansion. The Invar rods are encased in copper heat shields that act to keep the temperature of the spacer rods sufficiently uniform so that thermal expansion of the rods is negligible. The mirrors of the laser's resonant cavity are supported by the spacer rods in a manner such that the spacing between the mirrors is substantially invariant despite the thermal conditions in the laser's housing.
U.S. Pat. No. 4,099,143 describes a flowing gas laser having a gas tight hollow cyclindrical housing enclosing a blower, a heat exchanger, and means forming a discharge region, together with baffles and vanes for causing the gas to flow in a closed loop. The mirrors of the laser's resonant cavity are situated outside the housing and are supported by an optical bench having spacer rods that extend through the housing. To preserve the gas tight integrity of the housing while maintaining precisely accurate spacing between the mirrors despite the thermal expansion and contraction of the laser's housing, the mirrors of the resonant cavity are connected to the housing by bellows and the rods of the optical bench are arranged so that they are not affected by the thermal expansion and contraction of the housing.
The use of a separate bench to support the optical elements of the laser's resonant cavity has a number of drawbacks where weight, compactness, reliability, and cost of the laser are important considerations. In an arrangement, such as described in U.S. Pat. No. 4,099,143, where the rods of the optical bench extend through the laser's housing, the rods must be supported in a manner that preserves the gas tight integrity of the housing while allowing the housing to expand and contract without affecting the rods. The provision of reliable long lasting seals between the rods and the housing then becomes a problem. The rods and heat shields of the optical bench described in U.S. Pat. No. 3,808,553 occupy a considerable amount of space around the discharge cavity and add substantially to the complexity of the structure within the housing inasmuch as the rods must themselves be supported in a manner that does not affect their physical dimensions. The rods and their associated heat shields and support structure add an appreciable amount of weight to the laser. Further, the rods and their associated heat shields and support structure add to the cost of the laser both in material costs and in assembly costs.